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I have the D5100, I switched my off about a month ago (recommendation from this site for star trails) and i haven't switched it back one. I was just getting littled frustrated with the processing time on the camera itself taking so long approx 1 sec for every second of exposure, if required i do it PP. I can't comment on different it would make with lenes tho sorry

The only reason I asked was because I was scrolling through the Menu the other night and noticed it and prompted me to see whether any one used it or not, as I also have it switched off but haven't even thought about it since I had tried it out once but had heard not to worry about it so switched it off. So thought I'd pop the Question.

the main one is the usual NR we associate with removing colour noise on our images, and may be called High ISO NR. This will have options for various strengths of NR that can be applied .. ie. low/med/high or whatever it may be.

As already said, if you shoot raw, the NR applied can (sometimes) be changed in your raw editing software anyhow .. so your setting can be irrelevant.

Note tho, as you have a Nikon .. if you choose to use Nikon's free ViewNX2 to view or convert your raw files, then you can't alter the NR setting on the raw file. Annoying, but true .. and hopefully one day Nikon will update VNX2 to have some form of NR editing ability.
But if you use Nikon's (paid for) CaptureNX, then you can turn NR off completely on the raw file .. which removes the in camera NR application, and then you can add any NR to suit the image on an as needed basis.

I think many or most third party raw software may ignore the NR setting on raw files anyhow(not sure).

AND!!! there is another NR option in the camera which should be turned on for best effect.

This option is called Long Exposure NR(or LNR). This NR setting is different in that it doesn't just apply NR randomly, and only works on images over a specified exposure setting(I think 2sec and longer) ... and what this one does is to create an equally timed black exposure and then is applied to the real exposure to removed any hot pixel noise(which is different to short exposure colour noise).
The dark exposure is a specifically targeted NR routine that only removes noise on the pixels that have appeared coloured, where they otherwise shouldn't be.
This is otherwise known as dark frame subtraction, and is one of the best forms of NR available. Hence why it's usually recommended to leave it on .. as it's very accurate for the removal of noise on longer exposures.

With this LNR option tho, if you're doing long star trails and using multiple exposures to create a montage of an overall scene, it may be better to turn LNR off, to save a lot of time and battery power.
Because you have multiple images to overlay onto one another, the dark frame subtraction LNR is basically rendered unnecessary.

On the issue of dead pixels I heard someone talking about taking a exposure with the lens cap on and it should reveal any dead pixels haven't tried it yet but will be giving it a go out of interest. Have you tried this and what were your thoughts on this idea.

The other issue with long exposures used to be amp noise .. which manifests itself as a pinkish glow at the edges of the frame.
As this amp noise is usually slightly different from exposure to exposure, LNR used to be effective in minimising(but no eliminating completely) amp noise too.

Haven't seen amp noise in a camera for a long time now(D80 may have been the last Nikon camera with this annoyance )

Used it once very early on, had my Canon 50D setup on a star tracking telescope, shoot a 10 minute exposure of the milky way, it then took the NR a further 10 minutes after the exposure to do it's thing. Then I read/learn't from this site, RAW processing, and it's been off ever since.

....... it then took the NR a further 10 minutes after the exposure to do it's thing. ......

That'll be long exposure noise reduction(well, it sounds like it).

This is actually a very efficient and convenient way to process noise out of your long exposure images, if you don't exposure stack as the alternative. This is completely independent of what format is used, jpg, tiff or raw.
And while it sounds like a right ol PITA to have to wait a further 10mins for the dark frame to be processes, in general it's the better way to capture long exposures due to the nature of hot pixels.

The generally accepted rule that long exposure astro photographers use is to capture many shorter exposures and then combine them in software and the more exposures captured, the cleaner is the image from noise(hotpixels) in the end. But it still requires many minutes of image capture in total ... so in a sense of time management, exposure stacking or a single image with a dark frame .. it'll still basically take 20 odd minutes.

There are benefits and/or disadvantages for either method, and one apparent benefit of using the exposure stacking method if that it's supposed to produce a cleaner end result, but you still need to be capturing exposures for 20odd minutes. Whereas, with the single image/darkframe (LNR in camera) method, the advantage is that you capture one 10min exposure... pack up and move on(or go home), as the dark frame can be captured with the camera packed away, and even switched off!